<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    
</head>

<body>
    <div class="parent">
        <div class="left">
               <p style="height:800px;"></p>
        </div>
        <div class="right"></div>
    </div>
    <style>
        .parent {
            overflow: hidden;
        }

        .left,
        .right {
            padding-bottom: 9999px;
            margin-bottom: -9999px;
        }

        .left {
            background: #303;
            float: left;
            width: 100px;
        }

        .right {
            background: #c03;
            overflow: hidden;
        }
    </style>
</body>

</html>